I can also recommend Vit D3 drops for children, and lots of being outside to help with the sleep patterns.
Melatonin which can only be prescribed by a paediatrician I think is often helpful for getting children off to sleep.
Winding down routines for a child who is hyper stimulated/cannot switch off might not look like the same as the routine for a neurotypical toddler. A very very long bath for example with lots of sensory play and very little washing (more like being in a swimming pool) long before bedtime itself (my ds2 hated showers) a room with no toys in it, or bright colours, you lying in bed with him (we used an adult sized single so we could always get in when doing bedtime routine or reading a book to him) the same books, often quite long winded books with not much adventure or excitement but rhythm and familiar story. Lots of non fiction books on trucks and trains and lorries at bedtime, rather than fantasy stories. Ds2 also had luminous stars on the ceiling.carbs at bedtime, a snack like a banana and some toast, or milk. I also used to sing my children to sleep, mostly the same songs, nothing Disney or exciting, more LIttle Boy Blue or Boys and Girls Come out To Play, Nursery Rhyme songs. They didn't sing, they listened at bedtime.
In the day we did lots of singing and dancing and running around, so bedtime we avoided those things or playing or long drawn out stories. Ds2 (who has autism and now 17) woke v early (6am) but went to bed with this routine at about 7 or 8 pm. He usually woke once or twice in the night and co-slept at 3 but settled well in bed with us, and importantly went to bed in his own bed at 730-8pm.
If your little boy isn't getting much sleep he will be chronically tired and this will be affecting his impulse control as well as any underlying autism or adhd, hence the meltdowns. So working on his sleep routine will have incredible benefits, before you work on other stuff, he literally will not be able to control his responses if he is so tired, like you or I when we are knackered.
For us, it was exercise, fresh air, and sleep routines. Otherwise our lives were quite boringly samey, we didn't do many expeditions or do much which wasn't ordinary and local. Ds went to a Montessori Nursery for 3 hours in the morning in term time aged 3. Daycare would have been out of the question. He had a nap every afternoon for an hour and half at that age. We walked to nursery (half a mile) and back most days.